Just a word about playing progressives. If you use a standard 9/6 strategy, you can figure the EV fairly accurately for similar machines. The following table gives the expectation for the paying hands (using my basic 9/6 strategy -- other good strategies will be close):
Hand Payout Freq Contribution Pair J-A 1 21.44% 21.44% TwoPair 2 12.94% 25.88% Trips 3 7.44% 22.33% Straight 4 1.24% 4.49% Flush 6 1.10% 6.59% FullHouse 9 1.15% 10.36% Quads 25 0.23% 5.90% StrFlush 50 0.011% 0.548% RoyalFlush 800 0.0025% 2.013%
Let's consider a 7/5 Jacks machine with a progressive jackpot. You know that standard 9/6 Jacks has an optimal EV of 99.544%. On a 7/5 machine, you will lose 1.10% for the flush (6-5) and 2x1.15% for the full house. This reduces the EV to only 96.144%. To get to break even, you need 3.956% added to the Royal, or 800x3.956/2.013 = 1570 coins. Thus, on a $1 machine with 5 coins max, the jackpot break even is $5x(800+1570) = $11,850.
Of course, when the JP gets this big, major strategy variations become correct, which would lower the break even point considerably. However, I caution about guessing as to what variations you should make. Also, you should be aware that these variations will cost you quite a bit if you don't play long enough to hit the royal (for example, breaking up big pairs for 3 card royals is correct, but will cost you in the short run).